1. Field of the Invention
This invention relates to a process for making a fluorescent display device which is adapted to be used for an electronic apparatus, a vehicle, a game and the like, and more particularly to a process for making a tipless fluorescent display device which does not require a tip tube for evacuating an envelope of the device.
2. Description of the Prior Art
A fluorescent display device is generally constructed in a manner such that an envelope in which electrodes such as an anode, a control electrode, a cathode and the like are arranged has an evacuation tube of glass called a tip tube inserted at one end thereof through an evacuation hole of the envelope therein and outward projected at the other end thereof from the hole in order to facilitate the formation of the envelope of a high vacuum. The envelope is evacuated through the tip tube to a high vacuum and then the tip tube is sealed by melting to keep the envelope at a high vacuum.
However, such construction of the envelope has such a disadvantage that not only the projection of the tip tube from the envelope after the sealing of the tip tube deteriorates a space factor of a fluorescent display device in the incorporation of the fluorescent display device in a desired display system but also the tip tube is decreased in impact resistance because of being formed of glass. Thus, it was desired to develop a fluorescent display device without such a tip tube which is called a tipless fluorescent display device in the art.
Such a typical tipless fluorescent display device has been proposed in Japanese utility Model Publication No. 10291/1983 which includes an envelope in such a manner as shown in FIG. 1. More particularly, the conventional envelope, as shown in FIG. 1, comprises a glass substrate A and a casing H wherein the glass substrate A is formed with a through-hole B and has a ceramic member D fixedly mounted on the inner surface at the periphery of the through-hole B by means of frit glass C. The ceramic member D is also formed with a through-hole E of a smaller diameter so as to be substantially concentrical with the through-hole B of the glass substrate A, and the inner surface of the ceramic member D defining the through-hole E and the portion of the ceramic member D exposed to the through-hole B of the glass substrate A are applied thereto a metallizing layer F. Subsequently, the glass substrate A and casing H are hermetically secured together to form an envelope and then solder G is applied to the periphery of the through-hole E. Thereafter, the envelope is evacuated and finally the periphery of the through-hole E is heated to melt the solder G, to thereby hermetically seal the envelope.
Unfortunately, the conventional envelope assembled as described above has such a defect that the envelope is highly difficult in manufacturing and complicated in structure, because the ceramic member D hard to be worked must be formed with the through-hole E and it is required to securely fix the ceramic member D at the periphery of the through-hole B of the glass substrate A from the inside of the envelope in a specific atmosphere.
The envelope also has another disadvantage that the melting of the solder G to charge the through-hole E of the ceramic member D with the solder G causes the solder G to be in a liquid state to promote the generation of metal vapor from the surface thereof and gas due to the decomposition of organic flux used. This results in the vapor and gas remaining in the envelope and being adsorbed on an oxide cathode to cause the surface contamination of the cathode and/or the sintering of the contaminant into the cathode, to thereby deteriorate the electron emitting capacity of the cathode. Also, this causes the loss of vacuum in the envelope and the surface contamination of phosphor. Thus, the display characteristics of a fluorescent display device to be obtained is substantially decreased.
A further problem encountered with the conventional envelope is that, because a step of heating the substrate A and the casing H to hermetically bonding the both together and a step of subjecting the periphery of the through-hole E of the ceramic member D to a heating treatment to seal the through-hole while evacuating the envelope are required to be carried out at different places, respectively, adsorptive gas such as H.sub.2 O, CO.sub.2, CO and the like enters the envelope through the evacuation hole during the movement from the former step to the latter one to cause the loss of high vacuum in the envelope. Furthermore, a baking treatment for completely removing the adsorptive gas from the envelope is highly difficult.